Portable electrical equipment as represented by terminal devices of portable phones has been rapidly advancing over the last years, and expectations are high for these devices. The choice of image display device by these devices is a liquid crystal display device, which is thin and light-weight and low power consuming. Currently, terminal devices of portable phones use a liquid crystal display device of the simple-matrix variety. However, in anticipation of improved performance or much faster data communication speed, these devices are expected to be used in a variety of ways, such as portable TV telecommunications and Internet connection. For these purposes, a liquid crystal display device of the TFT active-matrix variety, which has a high display quality and fast response, is most suitable.
Here, the terminal device of the portable phone is commonly required to constantly display various information such as availability of communication, time, or remaining life of a battery, as shown in FIG. 8, even during standby in which no conversation or data communication is carried out. However, the power consumption of the liquid crystal display device during standby greatly influences the remaining life of a battery, and, in effect, influences the total standby time. It is known in industry that the terminal device of a portable phone, which does not allow the use of a large battery, with the total standby time of less than 200 hours is practically useless or unmarketable.
FIG. 9 shows one example of a relationship between power consumption of a liquid crystal display device and a total standby time of a terminal device of a portable phone. As can be seen from FIG. 9, the power consumption of the liquid crystal display device needs to be about 3 mW to have the total standby time of more than 200 hours. In terminal devices of portable phones currently available, the power consumption of the entire device is about 5 mW, whereas that of the STN liquid crystal display device of the simple-matrix variety is about 1 mW. This meets the foregoing requirement.
This is not the case for the liquid crystal display device of the TFT active-matrix variety, which consumes power in a range of 30 mW to several hundred mW, and the total standby time is only around 50 hours. This practically requires charging the battery every other day. It was therefore impossible to bring the liquid crystal display device of the TFT active-matrix variety for use as the liquid crystal display device which requires a constant display of various information even during standby.